1. Field of the Invention
This invention relates to an automatic controlling device of burners of a shaft furnace utilizing Fuzzy Logic. Conventional controlling method of a shaft furnace controls the burning power as a function of storage quantity of a holding furnace. The storage quantity has been divided into seven steps. The burning power has been prescribed to seven classes according to the seven steps of the actual stock.
2. Description of the Related Art
Conventional controlling system of a shaft furnace will now be explained by FIG. 1 which is a schematic view of this invention. A shaft furnace (1) is a tall cylindrical furnace having an inlet at the top and an outlet at the bottom. Material copper solid is supplied through the inlet into the shaft furnace. Several burners are furnished in the shaft furnace in order to heat and melt the material copper solid. Copper melt is exhausted from the shaft furnace through the outlet. Namely, a shaft furnace is a furnace to melt copper solid into copper melt. The shaft furnace is divided into three zones; zone A (lowest), zone B (middle), and zone C (highest). LNG burners A, B and C are respectively installed in the three zones A, B and C. LNG is liquefied natural gas. These burners use LNG as fuel. Air and LNG are supplied at the same time to the burners. The mixture ratio of LNG to air is always kept to be a most favorable, constant value by a gas supplier of the burners. The burner air pressure is an independent, controlling variable. But the LNG flux is a dependent variable which is determined by the air pressure, because the mixture ratio is kept to be constant. The burning power can be controlled by decreasing or increasing the burner air pressure. The shaft furnace (1) melts copper solid into copper melt by the LNG burners installed in the zones A, B and C.
The copper melt exhausted from the shaft furnace (1) is conveyed into a holding furnace (2). The holding furnace (2) is a cylindrical vessel which can rotate around an central axial line. The cylindrical vessel has an inlet at the center of a rear end wall through which the copper melt flows in the vessel. The cylindrical vessel has an outlet at a non-central spot of a front end wall at through which the copper melt flows out. The front end wall is slightly lower than the rear end wall. When the holding furnace (2) rotates around the central axis, the height of the outlet changes.
The rotation angle .theta. of the holding furnace (2) around the central axis is called inclination angle. Copper melt flowing from the outlet will be supplied to the casting process or the press-rolling process.
The height of the outlet of the front end coincides with the height of the copper melt in the cylindrical vessel. Thus, the storage quantity (H) of the copper melt contained in the vessel is uniquely determined by the height of the outlet. The height of the outlet is also determined by the inclination angle .theta.. Thus, the storage quantity (H) is determined by the inclination angle (.theta.).
Smaller inclination angle (.theta.) corresponds to larger storage quantity (H). Larger inclination angle (.theta.) corresponds to small storage quantity (H). Actual relation between the inclination angle (.theta.) and the storage quantity (H) depends on the geometric shape of the holding furnace (2). In practice, the relation of the inclination angle (.theta.) to the storage quantity (H) should be predetermined as a folding-line function. The function is able to be exhibited by the folding-line graph (.alpha.) in FIG. 1. The graph (.alpha.) has an abscissa showing the inclination angle .theta., and an ordinate showing the storage quantity H.
The storage quantity H is determined from the measured inclination angle .theta. by the graph (.alpha.). This is called conversion of inclination angle/storage quantity.
The flux of the copper melt flowing out of the holding furnace (2) is controlled to be nearly constant. Thus, large storage quantity (H) in the holding furnace (2) signifies large exhaust quantity yield flowing from the shaft furnace (1).
On the contrary, small storage quantity (H) in the holding furnace signifies small quantity of copper melt in the shaft furnace which melts solid material. This may mean insufficiency of the burning power or insufficiency of the supply of material copper solid.
In practice, it is desirable that the storage quantity (H) should be constant. If the storage quantity (H) varied, the quality of copper products which would be made by the later processes might also change. The fluctuation of the storage quantity (H) is likely to induce the fluctuation of the quality of products. Therefore, predetermining a preferable storage quantity (H.sub..quadrature.) which is called reference storage quantity, an operator compares the actual storage quantity (H) with the reference storage quantity (H.sub.58 ) and controls the air pressure of the burners of the three zones A, B and C so as to have the actual storage quantity (H) access to the reference storage quantity (H.sub..quadrature.).
Namely, the conventional controlling method has controlled the burning power by taking only the actual storage quantity (H) into account. When the actual storage quantity (H) was smaller, the operator increased the burning power in order to heighten melting of copper and to increase the yield of the copper melt in the shaft furnace. On the contrary, when the actual storage quantity (H) was larger, the operator decreased the burning power in order to suppress melting copper and decrease the yield of the copper melt.
Then, the operator determined a preferable burning power from graph (.beta.) which prescribed the relation of the storage quantity (H) to the burner air pressure, and controlled the air pressure of the burners. In practice, the storage quantity was divided into seven steps. Seven preferable values of burner air pressure were allotted to each of seven steps of the storage quantity (H). Thus, the relation between the storage quantity (H) and the burner air pressure (Q) was described by a stepwise function having seven steps. Graph (.beta.) in FIG. 1 shows the stepwise function.
Since this shaft furnace (1) is provided with three burners A, B and C, three values of the burner air pressure must be determined and controlled as functions of the actual storage quantity (H). In this example, the maximum storage quantity in the holding furnace was 15 ton. For example, preferable burner air pressures were settled to be 800 mmAq, 900 mmAq, and 950 mmAq for the burners A, B and C respectively where the storage quantity (H) is 8 ton to 9 ton.
The operator repeated such an adjustment of burner air pressure every sampling time with a certain length. "k" denotes the sampling number "H.sub.k ", "Q.sub.k " and "QA.sub.k " signify the storage quantity, inclination angle and air pressure of burner A at the k-th sampling time t.sub.k.
The same operation of adjustment repeated every sampling time. The adjusted values QA.sub.k, QB.sub.k and QC.sub.k of the air pressure of the burners A, B and C were entirely determined only by the actual storage quantity H.sub.k at the sampling time t.sub.k. No other values of H.sub.n at any other sampling times t.sub.n before t.sub.k (h&lt;k) were required for the adjustment of the air pressure QA.sub.k, QB.sub.k or QC.sub.k. The adjusted values of air pressure of the burners A, B and C were supplied to three controllers (3) and positioners (4). An output signal of the positioner (4) determined the opening degree of valves which controlled the air pressure of the burners. The output signal (7) of the positioner (4) was transmitted by an alternative switch (5) to a motor (6). According to the signal (7) of the positioner (4), the motor (6) rotated clockwise or counterclockwise in order to adjust the opening degree of a valve (8). A blower (9) inhaled and pumped air into the burners A, B and C of the shaft furnace (1). The valve (8) was provided with an air passage from the blower (9) to the burners. A feedback loop was formed to control every valve (8). The rotation angle of the motor (6) was detected as a valve-opening-signal (10) which was fed back to the positioner (4). If any deviation was detected between the actual opening degree and the designated one, the positioner (4) sent a revisional signal to the motor (6). In a short time, the opening degree of the valve (8) was settled at a designated one. In addition to the opening degree of the valve (8), the actual air pressure of the burners was also monitored to revise the output signal of the controller (3). By the action of the feedback loop, the air pressure of the burner coincided with the designated value QA.sub.k. Thus the burning power of the zone A was adjusted. Similar adjustment was carried out on the burners B and C of the zones B and C.
The conventional controlling system had some drawbacks. Since the conventional method took account only of the storage quantity in order to control the burning power of the shaft furnace, it could not discern whether the storage quantity was approaching to or receding from the designated (reference) value of storage quantity. The conventional method took no account of the direction of change of the storage quantity. It controlled the burning power to the same level if the actual storage quantity was the same, neglecting whether the storage quantity was increasing or decreasing. Thus, the control was not always effective. Sometimes the storage quantity would fluctuate due to the control. One matter which must be taken into account was a delay time. If the burning power was revised at the shaft furnace, it would take at least 10 minutes for the storage quantity to change. Such a long delay time sometimes put the controlling system out of order. Furthermore, the conventional controlling method took no account of the melt temperature T.sub.k of the holding furnace. Then manual adjustment of burning power was also indispensable to stabilize the melt temperature of the holding furnace, since fluctuation of the melt temperature would exert an undesired influence upon the copper products.